1. Field of the Invention
This invention relates to methods of and apparatus for bonding workpieces together wherein rapid and accurate alignment of workpieces to a bonding tip is achieved. In particular, the invention relates to systems wherein viewing of the workpieces and the tip to which alignment is to be achieved is performed through transparent elements on the bonding axes of machines used for making the bonds.
2. Description of the Prior Art
In apparatus used for bonding electronic devices such as integrated-circuit chips to substrates, one of the principal problems is aligning the integrated-circuit chips to bonding tips or apertures in compliant bonding members, then engaging the chips to the tips or apertures, and aligning the chips with a pattern on a substrate. A broad spectrum of optical systems have been utilized to facilitate such alignment. Most of the systems have some inherent limitations and are thus not entirely satisfactory.
One of the most common systems employs a half silvered mirror suspended between a bonding tip and a plane on which either chips or a substrate lie. An optical axis of a microscope is obliquely directed to the half silvered mirror and an image of both the tip and a chip or substrate on the plane over the mirror appear simultaneously in the microscope eyepiece. However, the oblique viewing causes perspective distortions and accurate alignment is difficult to achieve because of such distortions. Additionally, because the mirror is suspended between the tip and the workpiece with which the tip must be engaged, the mirror must be mounted on a swing-away mechanism which adds complexity to the bonding apparatus.
The oblique viewing problems associated with half silvered mirrors are eliminated to a certain extent through the use of beam-splitter prism combinations which are suspended similarly to the mirror but which direct an image of both a tip and a workpiece perpendicularly of a bonding axis on which it is suspended. These combinations however, require very accurate alignment to the bonding axis and thus the supporting mechanism, in addition to being capable of swinging away from the bonding axis, must have adjustments provided thereon to achieve precise planar alignment of the prism.
Systems employing reference reticles have also been used to accomplish the desired alingments. Such systems are described in U.S. Pat. No. 3,477,630 issued to F. J. Schneider on Nov. 11, 1969. These systems, while performing admirably when properly adjusted, do suffer from the limitation that accurate alignment of a mechanism which positions a movable head requires very precise adjustment with respect to the reference reticle in order to achieve the necessary degree of accuracy.
Even more complex systems have been utilized employing rapidly oscillated mirrors to project images of workpieces and bonding tips simultaneously to an operator while an operator tries to achieve alignment therebetween. Obviously, systems that use oscillating mirrors suffer from problems associated with inherent complexity.
Except for the reference reticle system, the above described systems have one common shortcoming. Some optical element of the apparatus must be at least temporarily suspended between the bonding tip and the workpiece. In order to provide space for these elements, the bonding tip must travel extensive distances in accomplishing either engagement with the workpiece for pickup purposes or for bonding purposes. Extensive travel of the bonding tip inherently carries with it difficulty in providing precise positioning of the bonding tip throughout its range of operation.